The application of HAZOP studies to integrated requirements models for control systems

This paper discusses the relationship between and the potential integration of three mature disciplines, hazard identification, structured and formal methods for control systems specification. In particular the methods used are Hazard and Operability Studies (HAZOPS), Ward and Mellor (W & M) Essential Models and the Calculus of Communicating Systems (CCS). Ward and Mellor models not only provide the required control flow and transformation extensions to capture the essence of control and protective and systems but we also suggest that the modelling philosophy, underpining the Ward and Mellor development method, provides an appropriate model to which the HAZOPS of such systems can be succesfully applied at the requirements stage. Specifically, we can demonstrate how the model can be effectively partitioned to provide the necessary vissibiloty of the systems requirements taht will enable an efficient and effective HAZOP meeting. The paper shows, with appropriate guide works and interpretations, how meaningful deviations from the required intent can be developed. We also show how the interpretation of the model can significantly effect our ability to produce meaningful deviations and what impact this has on the HAZOP process. A particular concern has been that these techniques should be seen as an extension of current good practice within the relevant engineering disciplines. Our approach is illustrated by a small but realistic industrial case study.